2025MNRAS.542.1969P

The origin of the high-$\alpha$ component of the Galactic bulge remains debated, unlike the bar-driven origin of the low-$\alpha$ bulge. We examine the metallicity-dependent dynamical properties of high-[Mg/Fe] stars in the bar region, using samples of low- and high-[Mg/Fe] stars from APOGEE DR17, complemented by the PIGS catalogue of ${\rm [Fe/H]}< -1$ stars. The mean Galactocentric rotational velocity $\overline{V}_{\phi }(R)$ is nearly cylindrical for both low- and high-[Mg/Fe] stars across the bulge and outer bar. $\overline{V}_{\phi }(R)$ of high-[Mg/Fe] stars with ${\rm [Fe/H]}\ge -0.6$ is similar within errors to low-[Mg/Fe] stars in the bulge, and 10-20 per cent lower in the outer bar. The mean radial velocity field of these stars exhibits a quadrupole pattern similar to low-[Mg/Fe] stars. Integrating orbits in realistic barred Galactic potentials, these model-independent properties correspond to a peanut bulge in the orbital density distributions for high-[Mg/Fe] stars with ${\rm [Fe/H]}\ge -0.6$, transitioning toward a more spheroidal structure at lower metallicities. Additionally, $\overline{V}_{\phi }({\rm [Fe/H]})$ for stars increases steeply as metallicity increases from about [Fe/H] $\sim -1.3$, resembling the spin-up observed at larger Galactic radii. This is accompanied by a transition in the dominant orbit families, from co- and counter-rotating ${\rm cloud\, A}$ and ${\rm x_4}$ orbits at low metallicities to co-rotating bar-supporting ${\rm x_1}$ family tree, ${\rm box}$, and ${\rm cloud\, A}$ orbits at solar metallicity. Our results strengthen the case that the bulk of the high-[Mg/Fe] component in the bar region evolved from an $\alpha$-enhanced disc, while metal-poor stars with ${\rm [Fe/H]}< -1$ trace a more turbulent origin.